Enhanced photocatalytic oxidation of free cyanide using hydrogen-treated TiO₂: effect of reduction temperature

Hydrogen-treated titanium dioxide (grey TiO₂) crystals were synthesized through a temperature-programmed reduction of commercial TiO₂ under an H₂/Ar mixed gas flow at elevated temperatures ranging from 600 to 800 °C. Analytical techniques, including Raman spectroscopy and x-ray Photoelectron Spectroscopy (XPS), were employed to probe the presence of oxygen vacancies in the grey TiO₂ and to track the variation in Ti³⁺ species concentration as a function of temperature. The reduced samples obtained at different temperatures were used as photocatalysts to oxidize free cyanide ions under UV light illumination. Among these, the photocatalysts reduced at 600 °C demonstrated superior activity, converting 30 ppm of free cyanide to cyanate ions within 120 min. This reaction time represents a significant enhancement in efficiency as compared to untreated TiO₂, which necessitated over 240 min to accomplish the same conversion. The improved performance of grey TiO₂ is ascribed to its elevated photocurrent and the positive shift in the flat band energy, which together accelerate electron transfer and limit electron-hole recombination rates.